1. Field of the Invention
The present invention concerns the protection of traffic streams on duplicated transmission paths. It is particularly, though not exclusively, applicable to digital data.
2. Description of Related Art
A well known technique for providing protection of transmitted data is the "Twin-path" technique in which one identical signal is sent over two paths continuously. At the receiving point a changeover switch selects one of the two incoming signals. The selection is normally based on some kind of signal monitoring such as the detection of gross alarms or of degraded performance such as high error rate. In such a system the changeover switch is a common element in the transmission scheme no matter which path has been selected. It therefore has to be designed for a high degree of reliability and is accordingly expensive.
Another well-known technique is to use one spare path to support 1 of N working paths, with an N-way changeover switch being required at both transmitting and receiving points. Again this is expensive, particularly where N may be required to be large, for example, 14.